With many ring field projection systems, the zone of best correction changes with wavelength. One attempt to bring this problem within tolerable limits involved using a narrow slit that included only a portion of the optimum zone for each wavelength. Another attempt to control the problem involved limiting the bandwidth of the exposing radiation. Still another attempt to control the problem involved constraining the design to provide adequate correction at all wavelengths in a given zone. It is an object of the present invention to control this problem in a new and improved manner, as will become apparent as the description proceeds.